Data write deferral during hostile events

ABSTRACT

Technology is disclosed for deferring storage operations (e.g., writes or reads) during hostile events. When a data storage device experiences a hostile event, e.g., a vibration, shock, etc. contact by a head of the data storage device with a disk surface can cause errors or indeed damage. The technology can cause a data storage device to suspend storage operations until the hostile event is no longer detected.

TECHNICAL FIELD

Several of the disclosed embodiments relate to data storage, and more particularly, to data storage architecture for enhanced storage resiliency during hostile events.

BACKGROUND

Commercial enterprises (e.g., companies) and others gather, store, and analyze an increasing amount of data. The trend now is to store and archive almost all data before making a decision on whether or not to analyze the stored data. Although the per unit cost associated with storing data has declined over time, the total costs for storage has increased for many companies because of the volumes of stored data. Hence, it is important for companies to find cost-effective ways to manage their data storage environments for storing and managing large quantities of data.

In response, data storage drive (e.g., magnetic hard disk drive) manufacturers have increased data storage capacity by using various innovative techniques, including increasing the number of platters and the density of tracks and sectors on one or both surfaces of the platters. A platter is commonly a circular disk having one or both sides of a rigid substrate coated with a magnetic medium, on which data is stored. Data storage devices typically have several platters that are mounted on a common spindle. Each side on which data is stored commonly has an associated read head and a write head, or sometimes a common read/write head. The platters are rotated rapidly within the data storage device about the spindle, and an actuator moves heads toward or away from the spindle so that data can be written to or read from tracks. A track is a circular path on the magnetic surface of the platters. One way of increasing data storage capacity is to have very narrow tracks and to place heads very close to the surface of the platter, e.g., micrometers (also, “microns”) away. However, because it takes more energy to write data than to read data (e.g., because the magnetic surface of platters must be magnetized to store data), data storage drive manufacturers inserted a buffer track between tracks storing data so that a wider track can be written to than read from. The buffer tracks could be magnetized when tracks on either side of the buffer tracks (“data tracks”) were written to, but read heads would only read from data tracks and ignore buffer tracks. However, buffer tracks decrease available space on platters.

To avoid wasting space on buffer tracks, a new technique employed by the industry is shingled magnetic recording (“SMR”). SMR is a technique to increase capacity used in hard disk drive magnetic storage. Although conventional data storage devices as described above record data by writing non-overlapping magnetic tracks parallel to each other, SMR involves writing new tracks that overlap part of the previously written magnetic track, leaving the previous track thinner, thereby allowing for higher track density. The SMR tracks partially overlap in a manner similar to roof shingles on a house. Shingled magnetic recording is expected to increase hard disk capacity by about 25% as compared to non-shingled storage.

However, if there exist hostile environmental events, e.g., vibrations, mechanical shocks, etc., use of SMR could lead to increased data corruption as compared to existing write techniques because not only could data in the track being written to become corrupt, but possibly also data in previously written adjacent tracks.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective plan view of a storage shelf and components therein, consistent with various embodiments.

FIG. 2 is a perspective view of a storage rack of storage shelves, consistent with various embodiments.

FIG. 3 is a perspective plan view of internal components of a data storage device, in accordance with various embodiments.

FIG. 4 is a block diagram illustrating components of a storage system, in accordance with various embodiments.

FIG. 5 is a flow diagram of a process of storing data, consistent with various embodiments of the disclosed technology.

FIG. 6 is a block diagram of a computer system as may be used to implement features of some embodiments of the disclosed technology.

DETAILED DESCRIPTION

Technology is described for deferring write operations to be performed by a data storage system when hostile environmental conditions are detected. In various embodiments, the technology includes one or more sensors, e.g., vibration sensors, that detect a hostile environmental event, e.g., a vibration. When the vibration is detected, e.g., exceeding a specified threshold, a processor associated with the data storage system suspends write operations, e.g., until the event is no longer detected. In various embodiments, the hostile environmental conditions can be detected using various sensors, e.g., vibration sensors, accelerometers, or even moisture, electo-magnetic-field, or other environmental sensors. In typical use, vibration sensors and/or accelerometers can detect movement. When the detected movement exceeds a specified threshold, a processor associated with the data storage system can command one or more data storage devices to suspend pending write operations. For example, the processor can command all data storage devices that are proximate to one another, e.g., co-located in a cabinet, shelf, or rack of a data storage system, to suspend the write operations. In another example, the processor can command individual data storage devices that are proximate to the sensor that detected a hostile condition. The data storage system can temporarily store (“log”) write operations it receives, e.g., in storage that is not susceptible to errors caused by vibration, e.g., in a system random access memory (e.g., non-volatile memory), solid state storage device, etc., e.g., while the write operations are suspended. After the environmental condition returns to a state not exceeding the threshold, e.g., an acceptable range, the data storage system can return to a normal state the data storage devices that had been commanded to suspend write operations, transfer the logged write operations to the data storage devices, and then continue to forward additional write operations to the data storage devices.

During the time that a data storage device has been commanded to suspend write operations, the data storage system may or may not be able to complete read operations. As an example, if the processor detects that there is a risk that any operation could result in physical damage to a data storage device, e.g., because a head may come in physical contact with a platter, the processor can suspend all operations. On the other hand, if the processor detects that there is little risk that a read physical operation could result in physical damage to a data storage device, but a greater than normal risk that a write operation could result in error, the processor can suspend write operations, but continue with read operations. Thus, the technology may have three states of operation and different threshold values for environmental conditions that determine in which state to operate: (1) normal; (2) read only; or (3) neither read nor write.

In various embodiments, the technology can receive at the data storage system a command to perform a write operation; cause a data storage device to perform a write operation; detect, during the write operation performed by the data storage device, a signal indicating that a vibration is detected; suspend the write operation until the signal stops; and continue, after the signal stops, the write operation. The technology can suspend the write operation if the detected signal exceeds a specified threshold. As an example, if the vibration detector detects a vibration that exceeds a specified threshold, a processor can suspend write operations directed to an affected data storage device. As another example, if the vibration detector detects a vibration that exceeds a different specified threshold, e.g., a higher threshold value, the processor can suspend read and/or write operations directed to the affected data storage device. The data storage device can be a high capacity magnetic data storage device, e.g., a data storage device that employs very high density tracks and/or a shingled magnetic recording technique. In various embodiments, the technology may be employable with other types of data storage devices, e.g., optical disc drives, magnetic tape drives that write multiple tracks to tapes, etc.

In various embodiments, the sensor that detects environmental conditions, e.g., a vibration detector, may be associated with a device and/or software that provides (“exposes”) an application program interface (also known as an “application programming interface,” or simply as “API”). As an example, data storage devices commonly include an interface to enable attached devices (e.g., the data storage system with which the data storage device is associated) to receive commands, return data, indicate status, etc. Data storage devices having (or modified to have) an environmental condition sensor (e.g., vibration sensor, accelerometer, etc.) can employ this interface to indicate the detected environmental condition. As examples, the interface can raise a flag, cause an interrupt, or simply return a value indicating the present condition. In various embodiments, the application program interface can indicate a value or range for the environmental condition or can receive threshold values and indicate that a specified threshold value has been exceeded. One having ordinary skill in the art would understand that the application program interface can have various combinations of properties and methods.

In various embodiments, the environmental condition sensor (also referred to herein as detector) can be associated with a shelf of a data storage system. Cabinets of data storage systems typically have multiple shelves and each shelf typically has multiple data storage devices. When a first shelf associated with a data storage system experiences a risky environmental condition but a second shelf does not, a processor associated with the data storage system can command all data storage devices of the first shelf to suspend read and/or write operations but not transmit that command to data storage devices of the second shelf. In various embodiments, the environmental condition sensor can be associated with an entire cabinet. In such embodiments, the data storage system may suspend operations to all data storage devices of the cabinet.

The storage devices of the storage system can be organized as storage shelves and storage racks, where each storage rack includes a number of storage shelves and each storage shelf includes a number of storage devices. The storage racks/shelves/devices can be distributed across various geographical locations.

Environment

FIG. 1 is a perspective plan view of a storage shelf 100 and components therein, consistent with various embodiments. The storage shelf 100 includes an enclosure shell 102 (partially shown) that encloses and protects multiple data storage devices 104. The data storage devices 104 may be hard drives, solid-state drives, flash drives, tape drives, or any combination thereof. It is noted that the term “enclose” does not necessarily require sealing the enclosure and does not necessarily require enveloping all sides of the enclosure.

The storage shelf 100 further includes control circuitry 106 that manages the power supply of the storage shelf 100, the data access to and from the data storage devices 104, and other storage operations to the data storage devices 104. The control circuitry 106 may implement each of its functions as a single component or a combination of separate components.

As shown, the storage shelf 100 is adapted as a rectangular prism that sits on an elongated surface 108 of the rectangular prism. Each of the data storage devices 104 may be stacked within the storage shelf 100. For example, the data storage devices 104 can stack on top of one another into columns. The control circuitry 106 can stack on top of one or more of the data storage devices 104 and one or more of the data storage devices 104 can also stack on top of the control circuitry 106.

In various embodiments, the enclosure shell 102 encloses the data storage devices 104 without providing window openings to access individual data storage devices or individual columns of data storage devices. In these embodiments, each of the storage shelves 100 is disposable such that after a specified number of the data storage devices 104 fail, the entire storage shelf 100 can be replaced as a whole instead of replacing individual failed data storage devices 104. Alternatively, the storage shelf 100 may be replaced after a specified time, e.g., corresponding to an expected lifetime.

The illustrated stacking of the data storage devices 104 in the storage shelf 100 enables a higher density of standard disk drives (e.g., 3.5-inch disk drives) in a standard shelf (e.g., a 19-inch width rack shelf). Each storage shelf 100 can store ten of the standard disk drives. In the cases that the data storage devices 104 are disk drives, the storage shelf 100 can hold the disk drives “flat” such that the spinning disks are parallel to the gravitational field.

The storage shelf 100 may include a handle 110 on one end of the enclosure shell 102 and a data connection port 112 (not shown) on the other end. The handle 110 is attached on an outer surface of the enclosure shell 102 to facilitate carrying of the storage shelf 100. The enclosure shell 102 exposes the handle 110 on its front surface. For example, the handle 110 may be a retractable handle that retracts to fit next to the front surface when not in use.

In some embodiments, the storage shelf 100 can include a sensor component 150 that is configured to detect environmental conditions and transmit indications of the detected environmental conditions, e.g., to a processor (not illustrated). Examples of the sensor component can include, e.g., a vibration sensor or detector, accelerometer, etc. The sensor component can include other electrical, mechanical, and/or electromechanical structures or sub-components, e.g., to sense hostile environmental conditions and communicate the detected conditions.

FIG. 2 is a perspective view of a storage rack 200 of storage shelves 220, consistent with various embodiments. The storage shelves 220 may be instances of the storage shelf 100 illustrated in FIG. 1. The storage rack 200, as illustrated, includes a tray structure 252 (e.g., a rack shelf) each securing four instances of the storage shelf 220. The tray structure 252 can be a standard 2U 19-inch deep rack mount. The storage rack 200 may include a stack of tray structures 252, each securely attached to a set of rails 262. Management devices 264 may be placed at the top shelves of the rack 200. For example, the management devices 264 may include network switches, power regulators, front-end storage appliances, or any combination thereof.

In some embodiments, the rack 200 can include a sensor component 250 that is configured to detect environmental conditions and transmit indications of the detected environmental conditions, e.g., to a processor (not illustrated). Examples of the sensor component 250 can include, e.g., a vibration sensor or detector, accelerometer, etc. The sensor component 250 can include other electrical, mechanical, and/or electromechanical structures or sub-components, e.g., to sense hostile environmental conditions and communicate the detected conditions.

FIG. 3 is a perspective plan view of internal components of a data storage device 300, in accordance with various embodiments. The data storage device can be a conventional hard disk storage device with the addition of a sensor component 350 that is configured to detect environmental conditions and transmit indications of the detected environmental conditions, e.g., to a processor (not illustrated). Examples of the sensor component 350 can include, e.g., a vibration sensor or detector, accelerometer, etc. The sensor component 350 can include other electrical, mechanical, and/or electromechanical structures or sub-components, e.g., to sense hostile environmental conditions and communicate the detected conditions.

The standard components of a hard disk data storage device are also illustrated: a front bezel 302 is attached to a mounting chassis 304. Also attached to the mounting chassis 304 can be a sealed chamber 306, an antivibration mount 318, a printed circuit board 314, and various electrical connectors 316. Although an “IDE” type connector is illustrated, one skilled in the art will recognize that other connector types are also currently employed, e.g., SATA, SAS, SCSI, and so forth. The sealed chamber 306 can include one or more disk platters 308, a head arm 310, and a head actuator 312. Other embodiments may include multiple of these components. Attached to the head arm 310 can be multiple read/write heads 320, e.g., one per readable/writable surface of the disk platter 308.

FIG. 4 is a block diagram illustrating components of a storage system 400, in accordance with various embodiments. The storage system 400 can include a data communication port 410 via which it receives various data communication commands. In some embodiments, the storage system 400 can include multiple data communication ports 410. A data processing module and or processor 402 can work in conjunction with an operating system 418 to analyze information received via the data communication port 410 or to be transmitted via the data communication port 410. The data processing module 402 may store data and instructions in an operational memory 406. The instructions can include an application program interface (API) 420. A storage controller 422 may function with the data processing module 402, via storage interfaces 414, to send commands to data storage devices 416, e.g., to store or retrieve data. This storing and retrieving may be performed in response to commands received via the data access API 420 or internal “housekeeping” commands. The storage system 400 can include a sensor component 450 that is configured to detect environmental conditions and transmit indications of the detected environmental conditions, e.g., to a processor. In various embodiments, the data processing module 402 can execute on the processor, or indeed be the processor. Examples of the sensor component can include, e.g., a vibration sensor or detector, accelerometer, etc. The sensor component 450 can include other electrical, mechanical, and/or electromechanical structures or sub-components, e.g., to sense hostile environmental conditions and communicate the detected conditions. In addition, or alternatively, the storage system 400 can receive indications of environmental conditions from remote sensors, e.g., sensors 150, 250, or 350. In some embodiments, the storage system 400 can include software and/or hardware components to temporarily suspend activities performed by the data storage devices 416, e.g., in response to hostile environmental conditions.

FIG. 5 is a flow diagram of a process 500 of storing data, consistent with various embodiments of the disclosed technology. In some embodiments, the process 500 may be implemented in environment 400 of FIG. 4, e.g., by sensor component 450 or by data processing module 402. The process 500 begins at block 502. At block 504, the process 500 detects a vibration level or other environmental condition (also, “hostile” condition). At decision block 506, the process 500 determines whether the detected vibration level exceeds a specified threshold (or the detected environmental condition indicates a hostile environment). If so, the process 500 continues at block 508. Otherwise, the process 500 continues at block 509. At block 508, the process 500 suspends writes at one or more data storage devices. The process then returns to block 504. At block 509, the routine 500 verifies recent writes to disk. As an example, after the hostile environmental condition ends, the routine 500 can verify that data previously attempted to be written was written correctly. If not, the routine 500 may cause data to be written from a log stored in non-volatile memory (not illustrated). At block 510, the process 500 permits writes at one or more data storage devices and then returns at block 512. As would be recognized by one having ordinary skill in the art, the logic described above can be rearranged, executed in parallel or in a thread, etc. Similar logic can be applied to suspend read operations when a vibration or other hostile condition is detected. In such a case, block 509 may be reconfigured to ensure that data that was nearby the head when the hostile condition was detected remains intact (e.g., to ensure that the head did not physically contact the disc and cause damage to the portions storing data).

In various embodiments, the system may take different actions depending on which of multiple data storage devices is affected by the hostile condition. For example, operations can be suspended for affected data storage devices, but not unaffected data storage devices. Alternatively, operations can be redirected from affected data storage devices to unaffected data storage devices, e.g., write operations or read operations when data is redundantly available. One skilled in the art will recognize that other variations are possible.

FIG. 6 is a block diagram of a computer system as may be used to implement features of some embodiments of the disclosed technology. The computing system 600 may be used to implement any of the entities, components or services depicted in the examples of the earlier figures (and any other components described in this specification). The computing system 600 may include one or more central processing units (“processors”) 605, memory 610, input/output devices 625 (e.g., keyboard and pointing devices, display devices), storage devices 620 (e.g., disk drives), and network adapters 630 (e.g., network interfaces) that are connected to an interconnect 615. The interconnect 615 is illustrated as an abstraction that represents any one or more separate physical buses, point to point connections, or both connected by appropriate bridges, adapters, or controllers. The interconnect 615, therefore, may include, for example, a system bus, a Peripheral Component Interconnect (PCI) bus or PCI-Express bus, a HyperTransport or industry standard architecture (ISA) bus, a small computer system interface (SCSI) bus, a universal serial bus (USB), IIC (I2C) bus, or an Institute of Electrical and Electronics Engineers (IEEE) standard 1394 bus, also called “Firewire”.

The memory 610 and storage devices 620 are computer-readable storage media that may store instructions that implement at least portions of the described technology. In addition, the data structures and message structures may be stored or transmitted via a data transmission medium, such as a signal on a communications link. Various communications links may be used, such as the Internet, a local area network, a wide area network, or a point-to-point dial-up connection. Thus, computer readable media can include computer-readable storage media (e.g., “non-transitory” media) and computer-readable transmission media.

The instructions stored in memory 610 can be implemented as software and/or firmware to program the processor(s) 605 to carry out actions described above. In some embodiments, such software and/or firmware may be initially provided to the computing system 600 by downloading it from a remote system to the computing system 600 (e.g., via network adapter 630).

The technology introduced herein can be implemented by, for example, programmable circuitry (e.g., one or more microprocessors) programmed with software and/or firmware, or entirely in special-purpose hardwired (non-programmable) circuitry, or in a combination of such forms. Special-purpose hardwired circuitry may be in the form of, for example, one or more ASICs, PLDs, FPGAs, etc.

Remarks

The above description and drawings are illustrative and are not to be construed as limiting. Numerous specific details are described to provide a thorough understanding of the disclosure. However, in some instances, well-known details are not described in order to avoid obscuring the description. Further, various modifications may be made without deviating from the scope of the embodiments. Accordingly, the embodiments are not limited except as by the appended claims.

Reference in this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the disclosure. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Moreover, various features are described which may be exhibited by some embodiments and not by others. Similarly, various requirements are described which may be requirements for some embodiments but not for other embodiments.

The terms used in this specification generally have their ordinary meanings in the art, within the context of the disclosure, and in the specific context where each term is used. Some terms that are used to describe the disclosure are discussed below, or elsewhere in the specification, to provide additional guidance to the practitioner regarding the description of the disclosure. For convenience, some terms may be highlighted, for example using italics and/or quotation marks. The use of highlighting has no influence on the scope and meaning of a term; the scope and meaning of a term is the same, in the same context, whether or not it is highlighted. It will be appreciated that the same thing can be said in more than one way. One will recognize that “memory” is one form of a “storage” and that the terms may on occasion be used interchangeably.

Consequently, alternative language and synonyms may be used for any one or more of the terms discussed herein, nor is any special significance to be placed upon whether or not a term is elaborated or discussed herein. Synonyms for some terms are provided. A recital of one or more synonyms does not exclude the use of other synonyms. The use of examples anywhere in this specification including examples of any term discussed herein is illustrative only, and is not intended to further limit the scope and meaning of the disclosure or of any exemplified term. Likewise, the disclosure is not limited to various embodiments given in this specification.

Those skilled in the art will appreciate that the logic illustrated in each of the flow diagrams discussed above, may be altered in various ways. For example, the order of the logic may be rearranged, substeps may be performed in parallel, illustrated logic may be omitted; other logic may be included, etc.

Without intent to further limit the scope of the disclosure, examples of instruments, apparatus, methods and their related results according to the embodiments of the present disclosure are given below. Note that titles or subtitles may be used in the examples for convenience of a reader, which in no way should limit the scope of the disclosure. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure pertains. In the case of conflict, the present document, including definitions will control. 

We claim:
 1. A method performed by a data storage system, comprising: receiving at the data storage system a command to perform a write operation, wherein the data storage system comprises at least two data storage devices that include a first data storage device and a second data storage device; causing at least one of the at least two data storage devices to perform the write operation; detecting, before or during the write operation, a signal indicating that a hostile event is detected; detecting that the signal affects the first data storage device but not the second data storage device; if the command to perform the write operation specifies the first data storage device suspending the write operation until the signal stops; continuing, after the signal stops, the write operation to the first data storage device; and if the command to perform the write operation specifies the second data storage device, not suspending the write operation.
 2. The method of claim 1, wherein the signal is detected in response to a result of the hostile event exceeding a specified threshold.
 3. The method of claim 1, wherein the at least two data storage devices are high capacity magnetic data storage devices.
 4. The method of claim 1, wherein the detecting includes invoking an application program interface for the at least two data storage devices.
 5. The method of claim 1, further comprising logging, while write operations are suspended, received commands in a non-volatile memory and after the signal stops, transferring the logged write operations to the first data storage device.
 6. The method of claim 1, wherein the first data storage device and the second data storage device are on different shelves of the data storage system.
 7. The method of claim 1, wherein the first data storage device and the second data storage device are on a same shelf of the data storage system.
 8. A data storage system, comprising: two or more shelves comprising at least two data storage devices that includes a first data storage device and a second data storage device; a sensor component configured to, detect vibration before or during a write operation; transmit a signal in response to detection of the vibration; and detect that the signal affects the first data storage device but not the second data storage device; a processor; and a machine-readable medium comprising machine executable code having stored thereon instructions executable by the processor to cause the data storage system to, if a command to perform the write operation specifies to the first data storage device, suspend the write operation while the vibration is detected; continue, after the vibration stops, the write operation to the first data storage device; and if the command to perform the write operation specifies the second data storage device, not suspend the write operation.
 9. The data storage system of claim 8, wherein the sensor component configured to detect the vibration is a sensor component of at least one of the first data storage device and the second data storage device.
 10. The data storage system of claim 9, wherein the at least one of the first data storage device and the second data storage device is configured to provide an application program interface.
 11. The data storage system of claim 10, wherein the application program interface comprises a method to indicate that the vibration is detected.
 12. The data storage system of claim 11, wherein the signal is transmitted if the vibration exceeds a specified threshold.
 13. The data storage system of claim 12, wherein the signal is an interrupt signal raised with the processor.
 14. The data storage system of claim 8, wherein the sensor component configured to detect vibration is a sensor component of a shelf of the data storage system.
 15. The data storage system of claim 8, wherein the sensor component configured to detect vibration is a sensor component of a cabinet of the data storage system.
 16. A non-transitory machine-readable storage medium comprising program code comprising instructions to: receive at a data storage system a command to perform a read or write operation, wherein the data storage system comprises at least two data storage devices that include a first data storage device and a second data storage device; cause at least one of the two data storage devices to perform the read or write operation; detect, during the read or write operation, a signal indicating that a hostile environmental event is detected; detect that the signal affects the first data storage device but not the second data storage device; if the command to perform the read or write operation specifies the first data storage device, suspend the read or write operation until the signal stops; and continue, after the signal stops, the read or write operation; and if the command to perform the read or write operation specifies the second data storage device, not suspend the read or write operation.
 17. The non-transitory machine-readable storage medium of claim 16, wherein the program code comprises instructions to log, while write operations are suspended, received commands in a memory and after the signal stops, transfer the logged write operations to at least one of the two data storage devices.
 18. The non-transitory machine-readable storage medium of claim 16, wherein the instructions cause a processor to invoke an application program interface for at least one of the two data storage devices.
 19. The non-transitory machine-readable storage medium of claim 16, wherein the program code comprises instructions to verify a previous write operation before continuing, after the signal stops, the read or write operation.
 20. The non-transitory machine-readable storage medium of claim 16, wherein the program code comprises instructions to: detect that the signal affects the first data storage device but not the second data storage device; and in an event that the command is directed to the first data storage device, redirect the read or write operation to the second data storage device.
 21. The data storage system of claim 8, wherein the first data storage device and the second data storage device are on different shelves of the data storage system.
 22. The data storage system of claim 8, wherein the first data storage device and the second data storage device are on a same shelf of the data storage system.
 23. The non-transitory machine-readable storage medium of claim 16, wherein the first data storage device and the second data storage device are on different shelves of the data storage system.
 24. The non-transitory machine-readable storage medium of claim 16, wherein the first data storage device and the second data storage device are on a same shelf of the data storage system.
 25. The method of claim 1, wherein the at least two data storage devices comprise Shingled Magnetic Recording storage devices.
 26. The data storage system of claim 8, wherein the at least two data storage devices comprise Shingled Magnetic Recording storage devices.
 27. The non-transitory machine-readable storage medium of claim 16, wherein the at least two data storage devices comprise Shingled Magnetic Recording storage devices. 